1. Field of the Invention
The present invention relates to a phase-change random access memory (PRAM) and more particularly, to a PRAM which has a low crystallization temperature, and quick operation speed by using a material having a short crystallization time.
2. Description of the Related Art
A phase-change random access memory (PRAM) uses a characteristic of a phase-change material, such as GeSbTe, wherein when an electrical pulse is applied to the phase-change material, a state of the phase-change material changes to crystalline and amorphous states according to heat generated in a localized area. A memory cell, which memorizes PRAM binary information, includes a phase-change layer, a bottom electrode contact (BEC) layer, and a switching device, such as a transistor.
A conventional PRAM includes a transistor, which is formed on a silicon wafer, a BEC layer, which is connected to a transistor source or a drain, and a phase-change layer. A conventional phase change layer is formed of a GST (GeSbTe) based material, on the basis of Ge2Sb2Te5 composition. The GST based material is called as chalcogenide, which is the same material as used in an optical recording apparatus, such as a DVD or a CD-RW. The BEC layer is formed in order to heat the phase-change layer. According to the extent of the heating of the phase-change layer, the phase of the conventional PRAM is changed to crystalline and amorphous states, and thus a resistance value changes according to the state of the conventional PRAM. Binary information can be stored and read since a current or voltage changes according to resistance of the phase-change layer.
Operation speed of a PRAM depends on phase-change speed between a crystalline structure and an amorphous structure of the phase-change layer. A GST material used in the conventional PRAM has a high crystallization temperature, and slow crystallization speed. Accordingly, when information is recorded from a set state of a crystalline structure to a reset state of an amorphous structure, a required reset current is high. Also, when information is recorded from a reset state of an amorphous structure to a set state of a crystalline structure, a required set time is equal to or more than 100 nsec, and thus it is difficult to realize a memory that operates in a high speed.
Accordingly, in order to realize a memory which has faster operation speed than a conventional resistance memory, a superior phase-change material should be used.